Within a nuclear reactor, the upper boundary of the reactor core is defined by the upper core plate. The upper ends of the nuclear reactor core fuel assemblies are detachably mounted to the undersurface of the upper core plates. The core contains fuel assemblies including fuel rods within which nuclear fuel pellets are disposed. Each fuel assembly has a plurality of tubes which receive nuclear reactor control rods for controlling the power output of the fuel assemblies and the reactor core. Movement of the nuclear reactor control rods is accomplished by control rod drive mechanisms through control rod drive shafts that extend through the pressure vessel.
The nuclear reactor upper support plate is vertically spaced above the upper core plate. An upper plenum chamber is defined between the upper support plate and the upper core plate. Reactor core coolant in the form of water is conducted through the upper plenum chamber for subsequent flow through the reactor core coolant loop and heat exchange system which is external of the pressure vessel and core barrel. The nuclear reactor control rods may be disposed within the upper plenum chamber when they are withdrawn vertically upwardly out of the core; when the control rods are lowered into the core their respective drive shafts are disposed within the upper plenum chamber. Protection and guidance for the control rods and their drive rods within the upper plenum chamber is provided with respect to the cross-currents of the flowing nuclear reactor core coolant by guide tubes. The guide tubes are interposed between, and fixedly connected to, the upper surface of the upper core plate and the upper support plate.
Annular flanges are provided at the lower ends of the guide tubes to secure the guide tubes to the upper core plate. Guide tube retaining pins position the guide tube flanges with respect to the upper core plate. The vertically disposed guide tube retaining pins have lower portions which are frictionally engaged within suitable bores defined within the upper core plate. The upper portion of each guide tube retaining pin is threadedly engaged with an internal hexagonal nut. Counterbored portions of the guide tube flange are engaged between a shoulder portion of the shank and the mated nut of the pin. To prevent retrograde rotation of the nut relative to the retaining pin wherein the nut could become disengaged from the upper portion of the pin, a dowel pin is passed through the nut and welded to a tab which is fixedly secured to the retaining pin.
This guide tube retaining pin and locking system positions the nuclear reactor control rod guide tubes within the upper core plate. However, in some reactors, stress corrosion cracking problems have developed within the retaining pins and weakened them to a point where they must be repaired or replaced by a welding operation. However, because the retaining pin and locking systems are disposed within operating plants and are located in an irradiated, underwater environment, remotely controlled welding operations are extremely difficult to achieve. The small structural components of the retaining pin and locking system, and the confined area within which the welding apparatus must be disposed and the welding operations must be performed contribute to this difficulty. Additionally, underwater welding operations entail high radiation exposure to personnel.
To fully appreciate the difficulties in performing such pin maintenance and repair operations, some understanding of the history of guide tube pin technology is necessary.
Since the inception of the RCCA guide tube concept by Westinghouse Electric Corp. in the mid-1960s, the design objective has been to "pin" the lower end of the guide tube to the upper core plate via two resilient "split pins" that are attached to the guide tube lower flange and which are engaged within circular holes in the core plate. These split pins have leaves that compress as they enter the upper core plate holes and provide a spring compression load to give the guide tube's lower end a degree of end fixity. This permits removal of the guide tube in the event of damage or excessive wear simply by unbolting the upper end where it is attached to the top support plate of the reactor internals and extracting it with a pull force sufficient to overcome the friction generated by the split pin leaf compression.
The material chosen for the split pins has been Inconel* X-750 because of its higher strength and superior mechanical properties (compared to stainless steel). its good wear properties, and the fact that its coefficient of thermal expansion is near that of stainless steel which in turn minimizes stresses caused by differential thermal expansion. The greater strength permits higher specified compressive loads in the leaves to achieve a higher degree of rigidity in the pinned end of the guide tube. In use, two split pins are provided in the lower flange of the guide tube and spaced 180.degree. apart to support the guide tube against the steady state flow and vibratory forces which act on the guide tube during normal plant operation, as well as to resist upset or abnormal loads applied to the tube which could occur during postulated pipe break accidents or earthquake conditions. The split in the two pins is opposed in direction so that each pin provides better restraint in a unique 90.degree. opposed direction. FNT * Inconel is a U.S. registered trademark owned by the International Nickel Corporation
In the late 1970's, stress corrosion began to develop in the Inconel X-750 pins in several nuclear plants. Significant time, money, and effort was spent arriving at a solution to the problem, and eventually, by January of 1988, approximately 60 nuclear facilities had the split pins removed and replaced with new Inconel X-750 split pins having advanced manufacturing and heat treating processes considered sufficient to produce pin longevity.
Unfortunately, Inconel X-750 pins of the new replacement design (which were not manufactured by Westinghouse) have also begun to manifest stress corrosion.
Clearly, there is a need for an improved guide tube pin that is capable of bearing the same shear load as an Inconel pin but which is not susceptible to stress corrosion. Such a pin should be rapidly and easily installable within a reactor core without major replacements to the upper internals and with a minimum of machining operations so as to minimize both the cost of installation and the radiation exposure of the workers. Finally, it would be desirable if the pin were made from relatively inexpensive and easily fabricated material having the same thermal expansion properties as the core plate.